1. Field of the Invention
This invention relates to encapsulation solvents for carbonless paper and in particular to carbonless paper having encapsulation solvents suitable for use in high speed electrophotographic printers and duplicators.
2. Description of the Related Art
Carbonless paper is widely used in the forms industry and carbonless paper forms have been printed in the past by conventional printing techniques such as offset printing, lithography, etc. With the advent of high speed electrophotographic copiers having dependable, high capacity collating systems and enhanced copy quality, there has been a movement to replace offset printing equipment located in print shops and large "quick-print" installations with electrophotographic copiers. For the successful use of carbonless papers in these copiers, compatibility of the carbonless paper with the machine is critical.
Carbonless papers are capable of producing an image upon application of pressure. They generally comprise at least two substrates (for example two sheets of paper) and involve coating one reactant, known as a color-former, on one substrate, and the other reactant, known as a developer, on another, mating, substrate. One surface, or side, of each substrate is coated with one of the two primary reactants. The two substrates are often referred to as a donor sheet and a receptor sheet. Means for preventing the reaction of the two reactants until activating pressure is applied are also provided. This is typically accomplished by encapsulation of one of the reactants. Preferably, the color-forming compound(s) in an appropriate hydrophobic solvent is encapsulated or contained in microcapsules and is coated on the back side of one sheet of paper to form a donor sheet. This donor sheet is then mated with a receptor sheet coated with a developer or reactant for the color-forming compound. The microcapsules serve the purpose of isolating the reactants from one another thus preventing reaction. Once activating pressure is applied to the untreated surface of the donor sheet, as from a stylus or business-machine key, the two substrates come into contact under sufficient pressure so that the capsules, corresponding to the pattern of applied pressure, rupture, and the solution of encapsulated color-former is released and transferred from the donor sheet to the receptor sheet. On the receptor sheet, a reaction between the previously separated reactants occurs. Since the color-former and the developer form a deeply colored image when reacted, an image forms on the receptor sheet corresponding to the path traveled by the stylus, or the pattern of pressure provided by the stylus or key. Herein the term, "activating pressure" includes, but is not limited to, pressure applied by hand with a stylus or pressure applied by a business machine key, for example a typewriter key; and the terms "encapsulation" and "encapsulated compounds" refer to microcapsules enclosing a color-former material therewithin.
A wide variety of processes exist by which microcapsules can be manufactured. These varied processes provide different techniques for producing capsules of varying sizes, alternative materials for the composition of the capsule shell, and various different functional materials within the shell. Some of these various processes are shown in U.S. Pat. Nos. 2,800,427; 2,800,458; 3,429,827; 3,516,846; 3,416,441; 4,087,376; 4,100,103; 4,909,605; and British Patent Spec. Nos. 1,046,409; and 950,443. A wide variety of capsule materials can be used in making the capsule shells, including gelatin and synthetic polymeric materials. A popular material for shell formation is the polymerization reaction between urea and formaldehyde, or melamine and formaldehyde, or the polycondensation products of monomeric or low molecular weight polymers of dimethylolurea or methylolated urea with aldehydes. A variety of capsule forming materials are disclosed, for example, in U.S. Pat. Nos. 2,800,458; 3,429,827; 3,156,846, 4,087,376; 4,100,103 and British Patent Spec. Nos. 1,046,409; 2,006,709 and 2,062,570.
A preferred construction comprises an encapsulated color-former dissolved in an appropriate hydrophobic solvent within microcapsules and coated with a suitable binder onto a back side of the donor sheet, sometimes referred to as a "coated back" (CB) sheet. A developer, also optionally in a suitable binder such as a starch or latex, is coated onto the front side of the receptor sheet sometimes referred to as a "coated front" (CF) sheet. The preparation of such a carbonless sheets is described by Matson in U.S. Pat. No. 3,516,846, incorporated herein by reference.
Constructions comprising a first substrate surface, on which is coated the encapsulated color-former, and, a second substrate surface, on which is coated a developer, are often prepared. The coated first substrate surface is positioned within the construction in contact with the coated second substrate surface. Such a construction is known as a "set" or a "form-set" construction.
Substrates, with one surface on which is coated the encapsulated color-former, and a second, opposite, surface on which is coated a developer can be placed between the CF and CB sheets, in a construction involving a plurality of substrates. Such sheets are generally referred to herein as "CFB" sheets (i.e., coated front and back sheets). Of course, each side including color-former thereon should be placed in juxtaposition with a sheet having developer thereon. CFB sheets are also typically used in form-sets. In some applications, multiple CFB sheets have been used in form-sets. These contain several intermediate sheets, each having a developer coating on one side and a coating with capsules of color-former on the opposite side.
Often carbonless paper is prepared and packaged in precollated form-sets in which sheets of various colors and surfaces are arranged opposite to their normal functional order. That is, the coated front sheet (CF) is first in the set and the coated back sheet (CB) is last with the required number of CFB sheets in between. This is done so that when the sheets are printed in a printer or copier which automatically reverses their sequence in the delivery tray, they will end up in the proper functional order for subsequent data entry. Sheets arranged in this manner are referred to as reverse sequence form-sets. In a second instance where reversal of the sequence in the delivery tray does not occur, the precollated sheets are arranged in their normal order. This arrangement is referred to as a straight sequence form-set. The type of sequenced form-set used for a particular printing operation is a function of the printing machinery.
The handling and transfer of the carbonless paper through the copier can lead to inadvertent rupture of capsules. Capsule rupture releases the encapsulation solvents from within the capsules, and results in exposure of the copier components to the solvent. Particularly sensitive copier components to solvent exposure are wires which serve the purpose of transferring electrical charges to photoconductor belts, copy paper or toner. The wires may be single wires or units commonly referred to as a corotron or a dicorotron. These wires are described in Davis et al., U.S. Pat. No. 4,086,650.
In the past, solvents used in the microcapsules of carbonless paper contained groups disposed toward breakdown in the atmosphere around a charging wire and contributed to unwanted residue build-up and contamination of the charging wire. Typically, contaminants build up on the charging wire and result in non-uniform current distribution across the charging wire. The non-uniform current distribution results in poor images being produced by the copy machine and/or machine difficulties.
Explanations for charging wire contamination is addressed by Williams. (see Edgar M. Williams, The Physics and Technology of Xerographic Processes, John Wiley and Sons). On page 71, Williams states "Normally, the atmosphere contains nitrogen, oxygen, oil vapors, Freon, salt crystals, dust, auto emissions, and a wide variety of elements and other chemicals. This air is ionized by the corona devices used in xerographic machines, so the possibility of interesting chemistry and crystal growth on and around corona wires is not surprising. Corona in air generates fair quantities of ozone so most commercial devices include activated charcoal filters to reduce ozone to acceptable levels. Ammonium nitrate salts can be created and precipitated by corona devices if the air contains ammonia at levels around 50 parts per billion. The salt crystallizes and grows on screen wires as well as on the PC surface. At high humidity, these salts become conductive and image quality is degraded because surface charge is transported laterally." The present invention addresses and minimizes the problems associated with contamination of the charging wires.
The chemistry used in carbonless papers is of two general types. In one type of carbonless paper, the image results from the reaction between an encapsulated leuco dye color-former and an acid developer. In another type of carbonless paper, the image results from the formation of a colored coordination compound by the reaction between an encapsulated ligand color-former a transition metal developer.
Leuco dye imaging chemistry employs capsules containing aliphatic hydrocarbon, or alkylated aromatic solvents. These solvents tend to have an odor, and upon inadvertent capsule rupture within a photocopier, a strong, objectionable, smell can result. Because copiers are often placed in areas with restricted ventilation, these odors can build up and cause discomfort to the machine operator.
Transition metal/ligand imaging chemistry usually involves capsules containing as the encapsulated ligand, derivatives of dithiooxamides (DTO), and as a developer, selected salts of nickel. Ligand/metal imaging systems have tended to use mixed solvents such as tributyl phosphate and diethyl phthalate. However, these solvents tend to decompose in the machine environment and contaminate the charging wires of the copier. This contamination eventually results in image deterioration and premature machine shutdown.
Both types of chemistry require solvents to dissolve the color-formers, and requirements for solvents for use in carbonless copy paper are stringent. For example, Okada et al., U.S. Pat. No. 4,699,658 give the requirements a solvent must fulfill.
1. It must dissolve the chromogenic dye precursor material at a high concentration.
2. It must not cause decomposition and color development of the chromogenic dye precursor.
3. It must have a high boiling point and not evaporate in the thermal drying step under high atmospheric temperature. (The requirement should be stated more broadly that the solvent must be stable to the encapsulation conditions.)
4. It must be insoluble in water.
5. It must show a high speed of color development and a high concentration of the developed color as well as high color stability after color developing.
6. It must be stable to light, heat, and chemicals.
7. The capsule fill should have a low viscosity so that it freely flows from the broken capsules.
8. It must be substantially odorless.
9. It must be safe and have a low toxicity.
10. It must be environmentally safe.
Okada et al. discuss solvent systems consisting of a mixture of biphenyls for use in a carbonless imaging system based upon a leuco dye color-former which is reacted with a phenolic resin developer. The advantages of Okada et al.'s solvents are that they permit rapid color development under low environmental temperatures and are taught to be substantially odorless.
One solution to the problems encountered in high speed copiers was achieved by Kraft and is disclosed in U.S. Pat. No. 4,906,605, incorporated herein by reference. Kraft found that the preparation of carbonless papers using high basis weight paper coupled with smaller capsule size and tighter capsule size distribution along with the elimination of stilt materials allows the successful use of these carbonless papers within copiers such as the Xerox 9000 series copiers and printers.
Many solvents have been used in carbonless paper constructions. For examples of some of the many solvents useful in carbonless imaging systems see Sandberg U.S. Pat. No. 4,596,996, column 2, lines 40-63. However, Sandberg does not distinguish among them with regard to particular usefulness, nor with the special requirements necessary for use in electrophotographic applications.
Brockett et al. U.S. Pat. No. 4,027,065 report that solvents for leuco dye systems which were both non-halogenated and non-aromatic had not yet found universal acceptance. They found that a high molecular weight ester, 2,2,4-trimethyl-1,3-pentanediol diisobutyrate, did not interfere significantly with color development and provided better fade resistance than solvents previously known.
Fraser U.S. Pat. No. 4,244,604 and Ludwig U.S. Pat. No. 4,461,496 teach the use of xylene, toluene, cyclohexane, phosphate esters, and phthalate esters as encapsulation solvents useful in carbonless papers employing ligand-metal imaging.
Miller, et al. U.S. Pat. No. 4,012,554 teach pressure rupturable microcapsules for use in a self contained paper. Their capsules contain all of the mark forming components in the same solution. Their imaging chemistry involves a leuco dye color-former reacting with an acidic developer such as a phenol. They disclose a solvent mixture containing a polar solvent which favors the uncolored form of the leuco dye. Upon imaging, evaporation of the polar component of the solvent mix results in a non-polar environment, favoring the colored form of the dye.
Recent improvements in solvents include the use of phenyl-sec-butylphenylmethane, as disclosed by Takashashi et al., U.S. Pat. No. 4,879,269. This system utilizes acid tripped leuco dye color-former chemistry for imaging.
A pigment such as carbon black and an adhesive dissolved in a solvent are disclosed by Okada et al. U.S. Pat. No. 4,696,856. The image is formed on a receptor sheet by transfer of the colored pigment, and the solvent is wicked away leaving the pigment in the adhesive. The solvent is used as a carrier for the adhesive and the pigment and there is no discussion of reactive chemistry used in an imaging process. They list solvents including xylene, toluene, ethylbenzene, mesitylene and other hydrocarbons. They also list hydrogenated aromatic hydrocarbons such as cyclohexane and esters such as diethyl phthalate, di-isopropyl phthalate, diethyl sebacate, diethyl adipate, ethyl benzoate, and the like.
To date, problems occurring with the electrophotographic copying of carbonless paper have not been adequately addressed. Charging wires becoming prematurely contaminated continues to hamper the use of carbonless paper in electrophotographic processes. It has now been discovered that the problems of residue build-up around charging wires which result in image deterioration and odors can be minimized through the use of specific solvents in the microcapsules.